linux-kernel - non-standard config in my running distro Default kernel: Effect?

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Date:	Wed, 17 Jun 2009 14:26:24 -0700
From:	Linda Walsh <lkml@...nx.org>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: non-standard config in my running distro Default kernel: Effect?

I found what appears to be a "non-standard configuration" in a
distro's "Default-SMP", non-Xen kernel, where /proc/config.gz
from the running kernel has (I only ran into this doing a diff between
a manually constructed config and my running kernel):

    CONFIG_X86_SMP=y
    CONFIG_X86_64_SMP=y

But does NOT have:

    CONFIG_USE_GENERIC_SMP_HELPERS=y

 From what I can tell, this shouldn't be a possible configuration, since
CONFIG_X86_SMP should force it to be set to 'y' (not unset).

Would this config, (SMP='y', generic_smp_helpers=(undef) ) have any
odd effects?  Is there a specific "effect" or "feature" one gets
by having 'smp=y' but not having the smp_helpers enabled?

In my x86_64 arch, the places I see it tested are:

block/blk-softirq.c:
    #if defined(CONFIG_SMP) && defined(CONFIG_USE_GENERIC_SMP_HELPERS)
    #else /* CONFIG_SMP && CONFIG_USE_GENERIC_SMP_HELPERS */
block/blk-sysfs.c:
    #if defined(CONFIG_USE_GENERIC_SMP_HELPERS)
include/linux/smp.h:
    #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
/kernel/Kconfig.hz:
    def_bool HIGH_RES_TIMERS && (!SMP || USE_GENERIC_SMP_HELPERS)
kernel/softirq.c:
    #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
    #else /* CONFIG_USE_GENERIC_SMP_HELPERS */

---
It appears it may disable some softirq routines.

In include/linux/smp.h, having it undefined would disable the definition 
of the functions:
  generic_smp_call_function_single_interrupt,
  generic_smp_call_function_interrupt,
  ipi_call_lock,      ipi_call_unlock,
  ipi_call_lock_irq & ipi_call_unlock_irq

Does this have any performance or integrity (lost interrupt)
implications?

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